There is publicly known a supercharging control device of an internal combustion engine including: a turbocharger of a variable displacement type in which air is supercharged by the use of an exhaust pressure of an internal combustion engine (engine); a variable nozzle capable of varying a turbine capacity of the turbocharger; and a variable-valve-timing controller for varying a valve opening characteristic of an intake valve for opening/closing an intake port of the engine and an exhaust valve for opening/closing an exhaust port of the engine (see, for example, JP-A-2006-348853).
The turbocharger of the variable displacement type is a turbocharger that includes: a compressor arranged in an intake passage of the engine; a turbine arranged in an exhaust passage of the engine; and a variable nozzle for varying a turbine capacity and that supercharges an intake air to be suctioned into a combustion chamber of a cylinder of the engine by the use of the pressure of an exhaust gas (exhaust pressure) generated in the exhaust passage of the engine and sends a compressed air supercharged in this manner into the combustion chamber of the cylinder of the engine.
In the turbocharger of the variable displacement type, in the case where the variable nozzle is controlled so as to throttle a scroll passage for introducing an exhaust gas into a turbine, the flow rate of the exhaust gas to be introduced into the turbine is more limited to a smaller value, that is, a turbine capacity is decreased. On the other hand, when the scroll passage is throttled, the flow speed of the exhaust gas to be introduced into the turbine can be increased, so that even if an engine speed is in a low speed range, a specified intake pressure (actual supercharging pressure) can be obtained.
Further, in the turbocharger of the variable displacement type, when the variable nozzle opens the scroll passage, the turbine capacity is increased. In this way, in a high speed range in which the flow rate of the exhaust gas to be introduced into the turbine is large, by fully opening the scroll passage by the variable nozzle, a larger actual supercharging pressure can be obtained.
The variable-valve-timing controller is constructed in such a way as to vary a period (a valve overlap amount) in which the intake valve and the exhaust valve are opened at the same time.
When the valve overlap amount is sufficiently large at the time of accelerating the engine, the supercharging control device of the internal combustion engine controls the opening of the variable nozzle in such a way that an intake port pressure (an actual supercharging pressure) is higher than an exhaust port pressure (an exhaust back pressure, an exhaust pressure), thereby improving a retard in supercharging at the time of accelerating the engine.
When the valve overlap amount of the intake valve and the exhaust valve is set at a specified value corresponding to a driving state of the engine, there is caused a phenomenon such that fresh air (intake air) blows off to the exhaust passage from the intake passage via the combustion chamber of the cylinder of the engine. When the phenomenon such that the intake air blows off in this manner is caused, the gas remaining in the combustion chamber, the volume of which usually corresponds to at least the space volume of the combustion chamber, can be pushed out or scavenged by the use of fresh air from the intake passage, whereby the gas in the combustion chamber of the cylinder can be replaced by the fresh air (which is referred to as “a scavenging effect”). In this way, the effect of improving the torque of the engine can be produced.
However, in a supercharging control device of an internal combustion engine in the related art, at an early stage of acceleration of an engine, for the purpose of rapidly increasing an actual supercharging pressure, a turbine capacity is decreased by closing a variable nozzle and a turbine output is rapidly increased by a rapid increase in an exhaust pressure which is caused by the turbine capacity being decreased. In this way, the actual supercharging pressure can be rapidly increased. However, in the case where the magnitude of decrease in the turbine capacity is large, a pressure difference between the exhaust pressure and the actual supercharging pressure is made large.
However, the intake valve and the exhaust valve are made to overlap each other, so that in the case where the exhaust pressure is higher than the actual supercharging pressure, the amount of remaining gas which remains in the combustion chamber of the cylinder of the engine is increased by the effect of an exhaust interference. In this way, although the actual supercharging pressure is rapidly increased at the early stage of acceleration of the engine by the rapid increase in the exhaust pressure which is caused by the decrease in the turbine capacity, an increase in the amount of intake air to be suctioned into the combustion chamber of the cylinder of the engine is made gentle, which hence presents a problem that an increase in engine output, that is, an increase in torque of the engine will be made gentle.